diff --git a/Makefile b/Makefile
index b60b806..f2ab6e3 100644
--- a/Makefile
+++ b/Makefile
@@ -9,7 +9,8 @@ all:
 	ocamlbuild $(TARGETS) $(DOC)
 
 benchs: all
-	ocamlbuild -use-ocamlfind -pkg bench -pkg unix tests/benchs.native
+	ocamlbuild -use-ocamlfind -pkg bench -pkg unix tests/benchs.native \
+		tests/simple_bench.native
 
 tests:
 	ocamlbuild -use-ocamlfind -pkg oUnit tests/run_tests.native
diff --git a/sequence.ml b/sequence.ml
index d322fe3..32e3f22 100644
--- a/sequence.ml
+++ b/sequence.ml
@@ -184,7 +184,9 @@ module MList = struct
   let rec push x l =
     if l.len = Array.length l.content
       then begin (* insert in the next block *)
-        (if l.tl == _empty () then l.tl <- make (Array.length l.content));
+        (if l.tl == _empty () then
+          let n = Array.length l.content in
+          l.tl <- make (n + n lsr 1));
         push x l.tl
       end else begin  (* insert in l *)
         l.content.(l.len) <- x;
diff --git a/tests/simple_bench.ml b/tests/simple_bench.ml
new file mode 100644
index 0000000..96611d7
--- /dev/null
+++ b/tests/simple_bench.ml
@@ -0,0 +1,11 @@
+
+open Sequence.Infix
+
+let _ =
+  let n = int_of_string Sys.argv.(1) in
+  let seq = 0 -- n in
+  let start = Unix.gettimeofday () in
+  seq |> Sequence.persistent |> Sequence.fold (+) 0 |> ignore;
+  let stop = Unix.gettimeofday () in
+  Format.printf "iter on %d: %.4f@." n (stop -. start);
+  ()